Thermoluminescence dosimetry, commonly referred to as TLD, is a technique for radiation dose measurement. In certain known materials, an incident flux of charged particles, such as beta rays, or uncharged particles, such as neutrons, or of electromagnetic energy, such as gamma rays and X-rays, creates charge carriers that are stored in traps within the material. When the materials are heated over time, the trapped charges are gradually released and, when released, they recombine to produce light emissions. When the emitted light is measured as a function of the temperature of the material over time, a glow curve is produced. The glow curve can be analyzed to determine the quantity of a particular flux to which a thermoluminescent material has been exposed. Typical materials used in TLD are fluorides of lithium and calcium, independently and mixed with other elements or compounds. Different materials respond to different kinds of fluxes so that a variety of thermoluminescent dosimeters may be manufactured for various applications.
TLD systems have been developed for monitoring the exposure of personnel who work in the vicinity of radioactive materials, X-ray equipment and the like, to particle and electromagnetic fluxes. Each person being monitored is given a badge to wear that is exposed to the same type and dosage of radiation as is the person wearing the badge. Although different types of badges have been used, generally the badges consist of an outer holder that houses a TLD card insert usually containing two, three or four thermoluminescent (TL) elements. Commonly, the card insert contains a hole for each TL element. A TL element is disposed approximately in the center of each hole with a relatively inert plastic material. Fluoropolymers, especially polytetrafluoroethylene, are commonly used for mounting TL elements in card inserts. The TLD cards, and the holders as well, may be provided with a machine readable code to enable automatic card and/or holder identification by TLD card reader.
Periodically the TLD cards are processed through a TLD card reader to produce an exposure record for each person being monitored. In the TLD card reader, the TL elements in each card are heated and the thermoluminescence as a function of TL element temperature is measured, for example by a photomultiplier. The photomultiplier response is processed electronically to provide a measurement of TL integrals and/or the glow curve. After a glow curve is determined, the TL elements are heated to anneal them so that they can be used again. The annealing cycle may preferably include several increasing and decreasing temperature cycles to erase any memory of the previous exposures to particle and energy fluxes.
Various TLD card readers heat the TL elements in a variety of different ways. In one common technique, a reader places a "hot finger" in direct contact with each TL element in a card insert. The heat produced by heating a finger is transferred to a TL element. While this technique provides good control of TL element temperature over time, the plastic material holding each TL element in an insert is also heated by the hot finger and can yield in response to mechanical pressure at elevated temperatures. This direct contact can result in mechanical damage to an insert card, shortening its life. In addition, the finger emits infrared radiation that can produce glow curve errors when a TL element has received a relatively low dose of radiation. The fingers must be advanced and retracted in an automatic reader so that mechanical failures are possible.
TL elements may be optically heated, for example, by shining a beam of coherent light on each element. However, specially designed, small and thin TL elements are required to respond to a light beam. The reduced mass yields a smaller amount of light for a given radiation dosage compared to more conventional TL elements. The smaller light emission means the threshold of dose detectability suffers, i.e. increases, in these elements. Although the threshold can be lowered somewhat by special electronic signal processing techniques, extra cost is incurred in threshold sensitivity compensation. These TL elements are typically mounted on an absorber that efficiency absorbs the energy in the light beam. These absorbers and their substrates can produce unwanted radiation that may be detected by a photomultiplier and introduce errors in a glow curve measurement.
TL elements that have been bonded to a susceptor, such as a graphite body, may be heated by induction with radio frequency (RF) energy. Like the optically heated TL elements, heating rates and TL element temperature as a function of time is difficult to control in RF heated elements. Relatively large amounts of RF power are needed to heat TL elements to the desired temperature. Since the temperature-time relationship is essential to accurate analysis of a glow curve, and to TL element treatments before and after glow curve generation, inability to control that relationship is of critical importance.
A flow of a constant temperature heated gas over TL elements as a heat source is also known. A gas flow eliminates mechanical contact with a TL element and moving parts. However, with a constant temperature gas source, a relatively high gas temperature must be maintained to achieve the desired highest temperature. The temperature variation of the TL elements over time, therefore, is not readily controllable when a constant temperature gas is used.
It is therefore desirable to provide a means for heating, in a controlled manner, each of a plurality of TL elements of the same or of different thicknesses mounted in a single card insert. With controlled heating of each TL element, their temperatures can be changed at preselected rates for detection of glow curves and for annealing any "remembered" exposure. In order to obtain long life of the TL elements, it is desired that the heating means avoid direct mechanical contact with the elements and that thermal glow of the heating means be eliminated or avoided.